Optical frequency standard frequency steering device and method based on diffuse reflection laser cooling atoms

Abstract

The application discloses a kind of based on diffuse reflection laser cooling atom's optical frequency standard frequency governing device and method, belong to laser cooling and quantum frequency standard technical field.The application is to solve the technical problem that existing diffuse reflection cold atom optical frequency standard is affected by the optical frequency shift introduced by cooling and repumping laser, resulting in limited long-term frequency stability, by periodically turning on or off cooling and repumping light source through time sequence control module, sampling and counting the error signal of probe light after passing through the cold atom group prepared by diffuse reflection by data acquisition and processing module during laser-off stage, calculating the deviation of local laser relative to cold atom transition frequency, and then realizing closed-loop calibration by frequency control unit driving frequency tuning unit.The application can effectively suppress the limitation of optical frequency shift on frequency stability, continuously improve the stability according to the law, while not interfering with the continuous output of the system, meet the needs of miniaturization, portability and long-time high-performance operation of optical frequency standard.

Description

Technical Field

[0001] This invention belongs to the field of laser cooling and quantum frequency standard technology, and particularly relates to a frequency control device and method for optical frequency standard based on diffuse reflection laser cooling of atoms. Background Technology

[0002] Since the invention of the laser, research on laser frequency stabilization has been ongoing, and the frequency stability of lasers has been continuously improved, enabling frequency-stabilized lasers to be used as optical frequency standards. However, when using hot atoms as the frequency reference for the laser, the frequency shift caused by the Doppler effect and collision effect of hot atoms is significant. Frequency-stabilized lasers (optical frequency standards) are easily affected by factors such as atomic velocity and atomic cluster temperature, limiting further optimization of frequency stability and frequency uncertainty.

[0003] The introduction of cold atom technology offers a new approach to improving the medium- and long-term frequency stability and uncertainty of optical frequency standards. Common methods for suppressing Doppler and collision shifts using cold atoms include cooling atoms / ions using optical lattices or ion traps. The best optical lattice clock has already achieved 10... -17 / The frequency stability of the best ion optical clock also reached 10. -15 / However, constructing optical lattices or ion traps typically requires more than five lasers, and probing narrow-linewidth clock transitions necessitates ultra-narrow-linewidth laser sources. While the currently mainstream Pound-Drever-Hall (PDH) technology can significantly narrow the laser linewidth, it also imposes extremely stringent requirements on cavity materials and the operating environment, resulting in a complex and expensive overall system. Therefore, the generation of optical frequency standards using optical lattice clocks and ion clocks remains primarily limited to laboratory environments.

[0004] Invention patent CN119696573A proposes a cold atom optical frequency standard based on long strip diffuse reflection cooling and its implementation method. This method combines laser-cooled atom technology with frequency modulation spectrum stabilization technology, utilizing three lasers (cooling, re-pumping, and narrow-linewidth detector) to achieve the fabrication of cold atoms and the generation of the optical frequency standard signal. This method eliminates the need for complex and expensive PDH systems, and the configuration of diffuse reflection laser cooling is simpler and more reliable compared to optical lattices and ion traps, effectively solving the aforementioned problems of optical lattice clocks and ion clocks. However, because the cooling and re-pumping lasers are close to the energy levels of the frequency-stabilized atoms, the final output of the optical frequency standard is limited by the power and frequency fluctuations of the cooling and re-pumping lasers due to the optical frequency shift introduced by the cooling and re-pumping lasers. This prevents the cold atom reference from achieving the expected long-term frequency stability. The advantage of regular descent. Therefore, further improving the problem of diffuse reflection cold atom optical frequency standards being affected by optical frequency shift, and realizing miniaturized, portable, and continuously operating cold atom optical frequency standards is the key research focus in the current technical field. Summary of the Invention

[0005] This invention aims to solve the technical problem in existing diffuse-reflection cold atom optical frequency standards where the frequency shift introduced by the cooling and re-pumping laser due to near-resonance limits the frequency stability and prevents the full utilization of the long-term stability advantage of cold atom references. It proposes a frequency driving device and method for optical frequency standards based on diffuse-reflection laser-cooled atoms, effectively avoiding optical frequency shift interference and ensuring that the system frequency stability increases with integration time. The performance of the optical frequency standard has been continuously improved, achieving high-performance, continuous, and portable operation.

[0006] To achieve the above objectives, the present invention adopts the following technical solution.

[0007] A frequency control device for optical frequency standard based on diffuse reflection laser cooling of atoms includes: Cooling laser source, re-pumped laser source, beam combiner, distribution assembly, long strip vacuum glass tube, detection laser source, first electro-optic phase modulator, first photodetector, laser phase detection module, data acquisition and processing module, frequency control unit, frequency tuning unit, and timing control module; The cooling laser source outputs a cooling laser, the re-pumped laser source outputs a re-pumped laser, and after being combined by the beam combiner, they are distributed into multiple laser outputs by the distribution component. The elongated vacuum glass tube is a hollow cylindrical structure. Except for the two ends, the outer surface of the glass tube is coated with a diffuse reflection coating. The side of the cylinder is provided with one or more incident windows for introducing the laser to prepare cold atomic clusters inside the glass tube. The probe laser source outputs a pre-stabilized local oscillator laser, which is divided into an output beam and a probe beam. The probe beam is phase-modulated by the first electro-optic phase modulator and passes through the long strip vacuum glass tube. After interacting with the cold atoms inside the tube, it is received by the first photodetector and a detection signal is generated. The laser phase detection module is used to generate a modulation signal and demodulate the detection signal to generate an error signal; The timing control module is used to control the switching timing of the cooling laser source and the re-pumped laser source, and synchronously control the data acquisition and processing module to sample and statistically analyze the error signal and calculate the frequency deviation during the laser shutdown phase. The frequency control unit drives the frequency tuning unit according to the frequency deviation to perform closed-loop tuning of the local oscillator laser frequency, so as to realize the frequency control of the local oscillator laser by cold atoms.

[0008] Furthermore, the detection laser source includes a detection laser, an isolator, a thermal atom gas cell, a second electro-optic phase modulator, a second photodetector, and a laser phase detection and high-speed servo control circuit, used to generate and pre-stabilize the local oscillator laser.

[0009] Furthermore, the distribution component includes a second polarizing beam splitter and a third polarizing beam splitter; a third half-wave plate and a fourth half-wave plate are provided on the laser path between the beam combiner and the distribution component to adjust the optical power distributed to each branch.

[0010] Furthermore, each laser output end of the distribution component is provided with an optical fiber coupling head, which is connected to a split-mode fiber to guide the laser to the incident window of the elongated vacuum glass tube.

[0011] Furthermore, the frequency control unit includes a voltage-controlled crystal oscillator, a second acousto-optic modulator, and a third acousto-optic modulator; the second acousto-optic modulator and the third acousto-optic modulator are disposed in the detection optical path, and their frequency shifting frequencies are the same but their directions are opposite.

[0012] Furthermore, it also includes a first acousto-optic modulator, a first reflector, a first half-wave plate, and a second half-wave plate; the first acousto-optic modulator is disposed on the output path of the cooled laser source; the first reflector, the first half-wave plate, and the second half-wave plate are respectively used to guide the laser into the beam combiner and adjust its optical power.

[0013] Furthermore, it also includes a fourth polarizing beam splitter for splitting the local oscillator laser into an output beam and a probe beam, and a fifth half-wave plate disposed in front of the fourth polarizing beam splitter; the probe laser source is also provided with a sixth half-wave plate and a seventh half-wave plate for adjusting the splitting ratio.

[0014] Furthermore, it also includes a second reflector, a third reflector, and a fourth reflector; the second reflector is used to reflect the phase-modulated probe beam into the elongated vacuum glass tube; the third and fourth reflectors are disposed inside the probe laser source and are used to guide the laser through the thermal atom gas chamber.

[0015] Furthermore, the thermal atom gas chamber inside the detection laser source is equipped with a temperature control structure and a magnetic shielding structure.

[0016] A method for frequency harnessing of optical frequency standards based on diffuse reflection laser-cooled atoms includes the following steps: 1) A frequency-stabilized cooling laser is output through a cooling laser source, and a frequency-stabilized re-pumped laser is output through a re-pumped laser source. The cooling laser and the re-pumped laser are combined by a beam combiner, and then distributed into multiple lasers by a distribution component and guided into a long strip-shaped vacuum glass tube. Cold atomic clusters are prepared by multiple reflections of the multiple lasers in the glass tube. 2) The local oscillator laser, which is pre-stabilized by detecting the output of the laser source, is divided into an output beam and a probe beam; 3) The probe beam is phase-modulated by the first electro-optic phase modulator and guided to pass through the cold atom cluster. The probe beam after passing through the cold atom cluster is received by the first photodetector to generate a detection signal. The laser phase detection module generates a modulation signal and demodulates the detection signal, thereby generating an error signal that reflects the deviation of the local oscillator laser from the transition frequency of the cold atom. 4) The timing control module controls the switching timing of the cooling laser source and the re-pumped laser source, and synchronously controls the data acquisition and processing module to sample and statistically analyze the error signal during the laser shutdown phase to calculate the frequency deviation; the frequency control unit drives the frequency tuning unit set on the local oscillator laser path according to the frequency deviation to perform closed-loop calibration of the local oscillator laser frequency, thereby realizing the frequency control of the local oscillator laser by cold atoms.

[0017] Furthermore, after the cooling laser output in step 1) is first subjected to acousto-optic modulation for red detuning frequency shift, it is then combined with the re-pumped laser through the first polarization beam splitter; the re-pumped laser is used to maintain the continuous cooling state of cold atoms in the long strip vacuum glass tube.

[0018] Furthermore, in step 1), the power distribution ratio of the multiple lasers distributed to each incident window is precisely controlled by adjusting the angle of the half-wave plate set at the front end of the distribution component.

[0019] Furthermore, the distribution component described in step 1) includes a multi-stage polarization beam splitter, and each laser output end is connected to a one-to-two multimode fiber to guide the laser to multiple incident windows on the sidewall of the elongated vacuum glass tube.

[0020] Furthermore, in step 2), the thermal atom gas chamber inside the probe laser source is used to lock the probe laser at the atomic transition frequency through modulation transfer spectrum stabilization technology, thereby obtaining a pre-stabilized local oscillator laser.

[0021] Furthermore, in step 3), before the probe beam passes through the cold atom cluster, it is first frequency-shifted by two acousto-optic modulators with the same frequency and opposite directions.

[0022] Furthermore, in step 3), the laser phase detection module filters, amplifies, and demodulates the detection signal output by the first photodetector to obtain the residual error signal reflecting the drift of the local oscillator laser relative to the cold atom resonance frequency.

[0023] Further, in step 4), the data acquisition and processing module converts the acquired error signal into a voltage deviation. The frequency control unit is a voltage-controlled crystal oscillator, which is used to convert the voltage deviation into a laser frequency deviation and feed it back to the acousto-optic modulator, which serves as a frequency tuning unit.

[0024] Furthermore, in step 4), a fixed duration is used as a complete operating cycle. The cooling and re-pumping light are turned on at the beginning of the cycle, and the light is turned off and error signal is sampled at the end of the cycle. The calculated frequency deviation is compensated to the frequency tuning unit at the beginning of the next cycle.

[0025] Compared with existing diffuse reflection cold atom optical frequency standards, the novelty and inventiveness of this invention are reflected in: This invention employs a probe laser source based on thermal atom modulation transfer spectrum stabilization as the local oscillator laser. It utilizes a long strip of cold atoms formed by diffuse reflection laser cooling as a frequency reference to detect and obtain a high signal-to-noise ratio cold atom frequency modulation spectrum. After pre-locking the local oscillator laser, it performs time-series feedback calibration. Because the laser cooling process in this embodiment is intermittent, compared to the continuous diffuse reflection cooling scheme in existing technologies, it effectively avoids the optical frequency shift problem introduced by the continuous presence of the cooling laser and the re-pumping laser. This significantly improves the frequency stability of the cold atom optical frequency standard during medium- and long-term operation, making its frequency stability exhibit a variation with integration time. A downward trend.

[0026] Furthermore, this invention maintains continuous operation of the hot atom local oscillator laser. The frequency calibration signal based on the cold atom transition spectrum is intermittently fed back to the control unit at the hot atom local oscillator laser output for closed-loop control when the cooling laser and re-pump laser are activated in the next cycle. This enables real-time and continuous output of the optical frequency standard system. This technical approach fundamentally overcomes the current technical bottleneck of limited frequency stability in optical frequency standards based on diffuse reflection cooling structures, providing a novel solution for achieving miniaturized, portable, high-performance, and long-term continuous operation of cold atom optical frequency standards, demonstrating significant technological advancement and practical application value.

[0027] Therefore, this invention, on the one hand, applies timing control to laser cooling, using cold atoms as a frequency reference during the shutdown phases of the cooling and re-pumping laser sources, effectively solving the optical frequency shift problem introduced by the cooling and re-pumping lasers; on the other hand, it separates the output of the hot atom local oscillator laser from the cold atom feedback calibration process, using the cold atom resonant transition as a reference, and feeding the frequency deviation back to the control unit at the output end of the hot atom local oscillator laser for calibration in a timing sequence, without interfering with the continuous output of the optical frequency standard system. This invention effectively overcomes the limitation of optical frequency shift on the improvement of long-term frequency stability in diffuse reflection cold atom optical frequency standards. The technical solution based on cold atom feedback to control the hot atom optical frequency standard has substantial technical progress and significant performance improvement effects, which have not been reported in existing domestic and foreign patent literature. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the optical frequency standard frequency control device based on diffuse reflection laser cooling atoms according to the present invention in the embodiment; Explanation of reference numerals in the attached figures: 1—Cooled laser source; 2—Re-pumped laser source; 3—Detection laser source; 4—First acousto-optic modulator; 5—First reflecting mirror; 6—First half-wave plate; 7—Second half-wave plate; 8—First polarizing beam splitter; 9—Third half-wave plate; 10—Second polarizing beam splitter; 11—First fiber optic coupler; 12—Second fiber optic coupler; 13—Fourth half-wave plate; 14—Third polarizing beam splitter; 15—Third fiber optic coupler; 16—Fourth fiber optic coupler; 17—Long, strip-shaped vacuum glass tube; 18—Second acousto-optic modulator; 19—Third acousto-optic modulator; 20—Fifth half-wave plate; 21—Fourth polarizing beam splitter; 22—First electro-optic phase modulator; 23—Second reflecting mirror; 24—First photodetector; 25—Laser phase detection module; 26—Timing control module; 27—Data acquisition and processing module; 28—Voltage-controlled crystal oscillator; 301—Detection laser; 302—Isolator; 303—Sixth half-wave plate; 304—Fifth polarizing beam splitter; 305—Seventh half-wave plate; 306—Sixth polarizing beam splitter; 307—Third reflecting mirror; 308—Hot atom gas chamber; 309—Seventh polarizing beam splitter; 310—Second photodetector; 311—Second electro-optic phase modulator; 312—Fourth reflecting mirror; 313—Laser phase detection and high-speed servo control circuit.

[0029] Figure 2 This is a schematic diagram illustrating the timing control of the cooling laser, the re-pumped laser, and the probe laser (local oscillator laser) based on the frequency stabilization of the thermal atom modulation transfer spectrum in the embodiment.

[0030] Figure 3 In this embodiment, the thermal atom saturated absorption spectrum detected by the second photodetector and the cold atom frequency modulation spectrum detected by the first photodetector are shown when the local oscillator laser is in scanning mode.

[0031] Figure 4 This is a flowchart illustrating the operation of the optical frequency standard frequency control device based on diffuse reflection laser cooling atoms in the embodiment. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0033] like Figure 1 As shown, this embodiment specifically proposes a frequency control device for optical frequency standard based on diffuse reflection laser cooling atoms, including: a cooling laser source 1, a re-pumped laser source 2, a probe laser source 3, a first acousto-optic modulator 4, a first reflector 5, a first half-wave plate 6, a second half-wave plate 7, a first polarizing beam splitter 8, a third half-wave plate 9, a second polarizing beam splitter 10, a first fiber optic coupler 11, a second fiber optic coupler 12, a fourth half-wave plate 13, a third polarizing beam splitter 14, a third fiber optic coupler 15, a fourth fiber optic coupler 16, a long strip vacuum glass tube 17, a second acousto-optic modulator 18, a third acousto-optic modulator 19, a fifth half-wave plate 20, a fourth polarizing beam splitter 21, a first electro-optic phase modulator 22, a second reflector 23, a first photodetector 24, a laser phase detection module 25, a timing control module 26, a data acquisition and processing module 27, and a voltage-controlled crystal oscillator 28. The detection laser source 3 includes: a detection laser 301, an isolator 302, a sixth half-wave plate 303, a fifth polarizing beam splitter 304, a seventh half-wave plate 305, a sixth polarizing beam splitter 306, a third reflecting mirror 307, a thermal atom gas cell 308, a seventh polarizing beam splitter 309, a second photodetector 310, a second electro-optic phase modulator 311, a fourth reflecting mirror 312, and a laser phase detection and high-speed servo control circuit 313.

[0034] The output frequency of the cooled laser source 1 is pre-stabilized to a selected cooling transition frequency using methods such as saturated absorption spectroscopy or modulation transfer spectroscopy. The cooling transition frequency refers to the light frequency absorbed when a particle transitions from a lower energy level to an upper energy level in the cooled transition. If the particle is a rubidium atom, the frequency is stabilized to the hyperfine transition frequency from the ground state F=2 to the first excited state F′=3. If the particle is a cesium atom, the frequency is stabilized to the hyperfine transition frequency from the ground state F=4 to the first excited state F′=5. After stabilization, the laser is frequency-shifted by the first acousto-optic modulator 4, producing a red detuning approximately two to three times the natural linewidth of the frequency-stabilized atomic transition. The frequency-shifted laser is called the cooled laser and is used to decelerate moving atoms within the elongated vacuum glass tube 17.

[0035] The output frequency of the heavy-pumped laser source 2 is pre-stabilized to a selected heavy-pumping transition frequency using methods such as saturated absorption spectroscopy or modulation transfer spectroscopy. The heavy-pumping transition frequency refers to the frequency of light absorbed when a particle transitions from the lower energy level of the heavy-pumping transition level to the upper energy level. If the particle is a rubidium atom, the frequency is stabilized to the hyperfine transition frequency from the ground state F=1 to the first excited state F′=2; if the particle is a cesium atom, the frequency is stabilized to the hyperfine transition frequency from the ground state F=3 to the first excited state F′=4. After stabilization, the laser is called a heavy-pumped laser and is used to maintain continuous cooling within the elongated vacuum glass tube 17.

[0036] The cooled laser, after being frequency-shifted by the first acousto-optic modulator 4, is reflected by the first reflector 5 and transmitted together with the re-pumped laser to the first polarization beam splitter 8 for beam combining. The optical power of the cooled laser and the re-pumped laser is adjusted by the first half-wave plate 6 and the second half-wave plate 7, respectively. The two combined laser beams output from the first polarization beam splitter 8 are then further split into four beams by the second polarization beam splitter 10 and the third polarization beam splitter 14, respectively. During this process, the splitting power of the second polarization beam splitter 10 and the third polarization beam splitter 14 is adjusted by the third half-wave plate 9 and the fourth half-wave plate 13, respectively. The four split laser beams are coupled into four 1-to-2 multimode optical fibers by the first fiber coupler 11, the second fiber coupler 12, the third fiber coupler 15, and the fourth fiber coupler 16, respectively. The fiber output ends yield eight laser beams containing both the cooled laser and the re-pumped laser.

[0037] The elongated vacuum glass tube 17 is a hollow cylinder, 1 meter long and 2 centimeters in inner diameter, filled with a thin layer of alkali metal atomic vapor. Its outer surface is coated with a diffuse reflection coating that has a high reflectivity of not less than 98% for both the cooling laser and the re-pumped laser. The two circular end faces of the vacuum glass tube 17 are not coated with the diffuse reflection coating, and eight entrance windows, each approximately 3 millimeters in diameter, are left uncoated on the cylindrical side surface to introduce the eight laser beams output from the four 1-to-2 multimode optical fibers. The eight laser beams are reflected multiple times within the tube, and under the radiation pressure of the cooling laser, the alkali metal atoms inside the elongated vacuum glass tube 17 are cooled, forming cold atom clusters that fill the entire length of the tube.

[0038] In other embodiments, the length and inner diameter of the elongated vacuum glass tube 17 can be selected according to actual needs, and are not limited to the above dimensions; the longer the tube length, the longer the interaction distance between the laser and the cold atoms, and the higher the signal-to-noise ratio of the cold atom transition spectrum; under the condition of local oscillator laser pre-locking and laser cooling operating in sequence, the voltage fluctuation of the acquired residual error signal is more accurate, which is beneficial to the feedback calibration of the optical frequency standard frequency driving device.

[0039] The power ratio of the combined cooled laser and re-pumped laser beam is not fixed and can be adjusted and optimized based on experimental results. The number of laser beams can be flexibly set according to the length of the vacuum glass tube and experimental requirements. By adding, removing, or replacing polarizing beam splitters and mirrors in the optical path, and by selecting multimode fibers with different splitting ratios (such as 1-to-1, 1-to-2, or 1-to-4), six, eight, or more laser beams can be output. The selection of the number of beams should balance the uniformity of the diffuse reflection field within the tube with the complexity of the system to achieve optimal cold atom fabrication results. For example, eight laser beams can be introduced into a 1-meter-long vacuum glass tube; six laser beams can be introduced into a 0.5-meter-long vacuum glass tube.

[0040] The laser output from the probe laser 301 in the probe laser source 3 is first transmitted to the isolator 302 to isolate the optical feedback in the rear optical path. After the isolator 302, it enters the fifth polarization beam splitter 304 and is split into two beams: one beam is used for frequency stabilization of the thermal atom modulation transfer spectrum, and the other beam is used as the final output laser of the probe laser source 3 based on the frequency stabilization. The splitting ratio of the split laser beam is adjusted by the sixth half-wave plate 303.

[0041] The laser used for frequency stabilization of the modulated transfer spectrum of hot atoms enters the sixth polarization beam splitter 306 and is split into two beams again. The splitting ratio is adjusted by the seventh half-wave plate 305 to obtain two laser beams with different strengths. The weaker beam serves as the probe laser for frequency stabilization of the modulated transfer spectrum. After being reflected by the third mirror 307, it passes through the hot atom gas cell 308 and then through the seventh polarization beam splitter 309, where it is received by the second photodetector 310. The stronger beam serves as the pump laser for frequency stabilization of the modulated transfer spectrum. It enters the second electro-optic phase modulator 311 for phase modulation. After being reflected by the fourth mirror 312 and the seventh polarization beam splitter 309, the modulated laser coincides with the probe laser in opposite directions within the hot atom gas cell 308, and together they interact with the atoms.

[0042] The laser phase detection and high-speed servo control circuit 313 generates a modulation signal applied to the second electro-optic phase modulator 311. The second photodetector 310 transmits the received thermal atom transition spectrum signal to the laser phase detection and high-speed servo control circuit 313, which performs filtering, amplification, and demodulation. The demodulated dispersive linear servo signal is then fed back to the fast feedback port and slow feedback port of the probe laser 301, achieving high-speed full-bandwidth locking of the probe laser 301. Finally, a narrow-linewidth, high-stability local oscillator laser with frequency stabilization based on the thermal atom modulation transfer spectrum is output. This local oscillator laser constitutes the core light source of the optical frequency standard frequency driving device of the present invention based on diffuse reflection laser cooling atoms.

[0043] The local oscillator laser, used as a probe laser source for frequency steering reference, can have its specific wavelength and stabilization frequency set according to the type of cold atoms used and the user's actual needs. Taking cold rubidium-87 atoms as an example, the probe laser source can be stabilized to a frequency of 5 in the 780nm band. 2 S 1 / 2 F=2 to 5 2 P 3 / 2 The F′=3 (F′=2, F′=1) transition can also stabilize the frequency to 420 nm band. 2 S 1 / 2 F=2 to 6 2 P 3 / 2 F′=3 (F′=2, F′=1) transitions, etc.; taking cold cesium atoms as an example, the detection laser source can be stabilized to 852 nm band 6 2 S 1 / 2 F=4 to 6 2 P 3 / 2 The F′=5 (F′=4, F′=3) transition can also stabilize the frequency to 455 nm band. 2 S 1 / 2 F=4 to 7 2 P 3 / 2 It features transitions such as F′=5 (F′=4, F′=3) and has good scalability.

[0044] The local oscillator laser output from the probe laser source 3 is further transmitted to the second acousto-optic modulator 18 and the third acousto-optic modulator 19 for frequency shifting. The second acousto-optic modulator 18 and the third acousto-optic modulator 19 have the same frequency shifting frequency but opposite shifting directions to ensure that the frequency of the laser after frequency shifting still corresponds to the atomic transition frequency. At the same time, the third acousto-optic modulator 19 also serves as the feedback control unit of the frequency driving device. The frequency-shifted laser enters the fourth polarization beam splitter prism 21 and is split into two beams: one beam serves as the final output laser of the optical frequency standard frequency driving device based on diffuse reflection laser-cooled atoms, and the other beam is used to detect the frequency modulation spectrum of cold atoms. The fifth half-wave plate 20 is used to adjust the power distribution ratio of the two laser beams. The laser used to detect the frequency modulation spectrum of cold atoms enters the first electro-optic phase modulator 22 for phase modulation. The modulated laser is reflected by the second reflector 23, passes through the cold atom cluster in the long strip vacuum glass tube 17, and is received by the first photodetector 24.

[0045] The laser phase detection module 25 generates a modulation signal applied to the first electro-optic phase modulator 22. When the local oscillator laser is in scanning mode, the cold atom transition spectrum signal received by the first photodetector 24 is filtered, amplified, and demodulated by the laser phase detection module 25 to obtain the cold atom frequency modulation spectrum. Through this spectrum calibration, the slope of the proposed locked transition line can be determined. When the local oscillator laser is in locked mode, the cold atom transition signal received by the first photodetector 24, after being filtered, amplified, and demodulated by the laser phase detection module 25, manifests as a residual error signal that varies with time. This error signal reflects the deviation of the local oscillator laser relative to the cold atom transition frequency.

[0046] The laser phase detection and high-speed servo control circuit 313 and the laser phase detection module 25 include functions such as generating radio frequency modulation signals, demodulating, filtering, and amplifying atomic transition spectrum signals; among them, the laser phase detection and high-speed servo control circuit 313 also has the function of high-speed servo feedback control of the laser frequency. The laser phase detection module 25 transmits the demodulated residual error signal to the data acquisition and processing module 27, which performs timing sampling and statistical processing on the voltage value of the residual error signal, such as... Figure 1 As shown by the dashed line.

[0047] After the local oscillator laser is pre-locked, the cooling laser is set to run in a timed sequence with a cycle of 500 milliseconds. The timing control module 26 controls the cooling laser source 1 and the re-pumped laser source 2 to be turned on for the first 470 milliseconds of each cycle to prepare cold atoms; the cooling laser source 1 and the re-pumped laser source 2 are turned off for the next 30 milliseconds. At the same time, the data acquisition and processing module 27 acquires the voltage value of the remaining error signal at a frequency of once per millisecond. Using the average value of the 30 voltage data acquired in the first timed cycle as the reference value, in each subsequent cycle, the average voltage value of the acquired remaining error signal is compared with the reference value, and the obtained voltage deviation is transmitted to the voltage-controlled crystal oscillator 28.

[0048] The 500-millisecond timing period set in this embodiment can be appropriately adjusted based on the inherent frequency stability performance of the thermal atom local oscillator laser. Theoretically, if the integration time corresponding to the inflection point (i.e., the upward inflection point) of the Allan variance curve of the local oscillator laser is used as the timing period, better medium- and long-term frequency stability can be obtained. However, in actual settings, it is also necessary to ensure that cold atom clusters can be fully formed within the period. Therefore, the lower limit of this period must take into account both cold atom trapping and preparation efficiency. After the cooling laser source and the re-pumping laser source are turned off, the formed cold atom clusters will maintain an effective existence time of about 20 to 30 milliseconds under ideal conditions without external interference. Therefore, the window time used for data acquisition in the timing period (i.e., the cooling light off time) can also be flexibly set according to the cold atom lifetime in actual applications to ensure the quality of feedback data.

[0049] The voltage-controlled crystal oscillator 28 uses the pre-calibrated slope of the cold atom frequency modulation spectrum under the local oscillator laser scanning state as the voltage-frequency conversion coefficient. The frequency deviation of the local oscillator laser relative to the first cycle is calculated by dividing the voltage deviation by this conversion coefficient. The frequency deviation obtained in the Nth cycle (N≥2) is fed back and superimposed onto the fixed modulation frequency of the third acousto-optic modulator 19 at the start of the N+1th cycle, achieving closed-loop calibration of the local oscillator laser frequency. Therefore, the medium- and long-term frequency stability of the optical frequency standard frequency driving device based on diffuse reflection laser-cooled atoms is... The trend continues to rise.

[0050] This embodiment also provides a method for controlling the frequency of an optical frequency standard based on diffuse reflection laser cooling atoms, which specifically includes the following steps: 1) The frequency-stabilized cooled laser source 1 output laser is frequency-shifted by the first acousto-optic modulator 4 to generate a cooled laser with a red detuning of approximately two to three times the natural linewidth of the atomic transition relative to the stabilization frequency. This cooled laser is reflected by the first reflector 5 and combined with the frequency-stabilized re-pumped laser source 2 output laser by the first polarization beam splitter 8. The optical power of the cooled laser and the re-pumped laser is adjusted by the first half-wave plate 6 and the second half-wave plate 7, respectively.

[0051] 2) The two combined laser beams output from the first polarization beam splitter 8 are reintroduced into the second polarization beam splitter 10 and the third polarization beam splitter 14, splitting the laser beams into four beams. The splitting power of the second polarization beam splitter 10 and the third polarization beam splitter 14 is adjusted by the third half-wave plate 9 and the fourth half-wave plate 13, respectively. The four split laser beams are then coupled into four 1-to-2 multimode optical fibers via the first fiber coupler 11, the second fiber coupler 12, the third fiber coupler 15, and the fourth fiber coupler 16, respectively.

[0052] 3) A diffuse reflection coating with high reflectivity for cooling lasers and re-pumping lasers is coated on the outer surface of the cylindrical sidewall of the elongated vacuum glass tube 17. Eight uncoated small holes are reserved as laser incident windows. The eight laser beams output from the four optical fibers in step 2) are incident through the eight windows respectively, undergoing multiple reflections inside the elongated vacuum glass tube 17. Under the action of radiation pressure, the alkali metal atoms inside the tube are cooled, filling the entire 1-meter-long glass tube with cold atomic clusters.

[0053] 4) The laser output from the probe laser 301 passes through the isolator 302 and then sequentially through the sixth half-wave plate 303 and the fifth polarization beam splitter 304. After adjusting the splitting ratio, it is split into two beams: one beam is used for frequency stabilization of the thermal atom modulation transfer spectrum, and the other beam serves as the final output of the probe laser source based on this frequency stabilization. The laser used for frequency stabilization continues to pass through the seventh half-wave plate 305 and the sixth polarization beam splitter 306 before being split again. The weaker beam passes through the thermal atom gas chamber 308 (equipped with temperature control and magnetic shielding structure) and is received by the second photodetector 310. The stronger beam enters the second electro-optic phase modulator 311 for phase modulation and then coincides with the first beam in the opposite direction as it passes through the thermal atom gas chamber 308.

[0054] 5) The thermal atom transition spectrum signal received by the second photodetector 310 is sent to the laser phase detection and high-speed servo control circuit 313 for filtering, amplification, and demodulation. The demodulated dispersive linear servo signal is then fed back to the fast feedback port and slow feedback port of the probe laser 301, respectively, to achieve high-speed full-bandwidth frequency locking, thereby obtaining a narrow-linewidth, high-stability local oscillator laser output. The laser phase detection and high-speed servo control circuit 313 simultaneously generates a modulation signal applied to the second electro-optic phase modulator 311 and has the functions of demodulating, filtering, and servo feedback control of the thermal atom transition spectrum.

[0055] 6) The local oscillator laser obtained in step 5) is sequentially fed into the second acousto-optic modulator 18 and the third acousto-optic modulator 19 for frequency shifting. The two acousto-optic modulators have the same frequency shifting frequency but opposite shifting directions, thus maintaining atomic transition resonance while achieving frequency control. The frequency-shifted laser is then sequentially split into two beams by the fifth half-wave plate 20 and the fourth polarization beam splitter 21: one beam serves as the final laser output of the frequency driving device of this invention, and the other beam is fed into the first electro-optic phase modulator for phase modulation. The third acousto-optic modulator 19 also serves as the feedback control unit for frequency closed-loop calibration in step 11).

[0056] 7) The phase-modulated laser passes through the cold atom cluster in the long vacuum glass tube 17 and is received by the first photodetector 24. When the local oscillator laser is in scanning mode, the cold atom transition spectrum signal collected by the first photodetector 24 is filtered, amplified, and demodulated by the laser phase detection module 25 to obtain the cold atom frequency modulation spectrum and calibrate the slope of the proposed locked transition spectrum. When the local oscillator laser is in locked mode, the first photodetector 24 outputs a residual error signal that fluctuates with time, reflecting the small deviation of the local oscillator laser relative to the cold atom transition frequency. The laser phase detection module 25 can also generate a modulation signal applied to the first electro-optic phase modulator 22, and has the functions of demodulation, filtering, and amplification of the cold atom transition signal.

[0057] 8) After the local oscillator laser is pre-locked, with a complete cycle of 500 milliseconds, the timing control module 26 controls the cooling laser source 1 and the re-pumped laser source 2 to be turned on for the first 470 milliseconds of each cycle to prepare cold atoms, and turns off the above laser sources for the last 30 milliseconds to collect error signals. The data acquisition and processing module 27 collects the remaining error signal voltage value at 1-millisecond intervals during the last 30 milliseconds of each cycle.

[0058] 9) The average value of the 30 residual error signal voltage data collected in the first cycle is used as the reference value. The average of the 30 data points in the subsequent Nth cycle (N=2, 3, 4, ...) is then compared with the reference value to obtain the relative voltage deviation. The timing control module 26 can be implemented using LabVIEW or other control software. The cycle parameter is related to laser stability, and the laser source shutdown time can also be adjusted according to the lifetime of the cold atom cluster.

[0059] 10) Input the relative voltage deviation obtained in step 9) into the voltage-controlled crystal oscillator 28 and convert it into a frequency deviation. The voltage-controlled crystal oscillator 28 uses the cold atom frequency modulation spectrum slope pre-calibrated in step 7) as the voltage-frequency conversion coefficient, and obtains the frequency deviation by dividing the voltage deviation by this conversion coefficient.

[0060] 11) The frequency deviation calculated in the Nth cycle is fed back and superimposed onto the fixed modulation frequency of the third acousto-optic modulator 19 at the beginning of the N+1th cycle to achieve closed-loop frequency calibration. Through continuous time-series operation, the medium- and long-term frequency stability of the frequency driving device of this invention is ultimately achieved. The trend continues to improve.

[0061] The principle behind the deceleration of moving atoms by cooling lasers lies in the following: when the frequency of the cooling laser is detuned relative to the atomic transition frequency, due to the Doppler effect, moving atoms tend to absorb photons propagating in the opposite direction of their motion. This generates radiation pressure in the opposite direction of atomic motion during momentum exchange between the atom and the photon, forming an effective damping force that slows down the atom's velocity. This mechanism is the basic principle of Doppler cooling, and the relevant theoretical details will not be elaborated upon in this invention.

[0062] In practical applications, the cooling effect is better when the frequency of the cooling laser has a red detuning of approximately 2 to 3 times the natural linewidth of the atomic transition relative to the atomic transition frequency. Taking typical alkali metal elements as examples, the natural linewidth of the D2 transition line from the ground state to the first excited state of rubidium atoms is approximately 6.1 MHz, corresponding to a red detuning of approximately 12 MHz to 18 MHz for the cooling laser; the natural linewidth of the D2 transition line of cesium atoms is approximately 5.2 MHz, corresponding to a red detuning of approximately 10 MHz to 15 MHz.

[0063] During the cooling process, some atoms in the cooling transition lower energy level (e.g., the F=2 state) transition to the excited state after absorbing the cooling laser. Subsequently, they may spontaneously emit radiation to another ground state energy level that does not participate in the cooling cycle (e.g., the F=1 state), thus losing their response to the cooling laser and being unable to continue participating in the cooling process, resulting in a decrease in cooling efficiency. To solve this problem, this invention introduces a re-pumped laser into the cooling light field, with its frequency tuned to the transition frequency from the uncooled ground state energy level to the excited state energy level. This allows atoms to be re-excited from this "dark state" to the energy level that participates in the cooling cycle, thereby restoring their response to the cooling laser and maintaining a stable and effective cooling cycle.

[0064] like Figure 2 As shown, this embodiment demonstrates how the optical frequency standard frequency driving device operates the laser cooling process using a timing control method. The entire timing cycle is set to 500 milliseconds, with the cooling laser source 1 and the re-pumped laser source 2 activated for the first 470 milliseconds of each cycle to prepare cold atoms, and the cooling laser source 1 and the re-pumped laser source 2 deactivated for the last 30 milliseconds to collect the remaining error signal voltage of the system. Simultaneously, the probe laser source 3 (i.e., the local oscillator laser) remains continuously operational, providing the system with a consistently stable contrast frequency input.

[0065] like Figure 3As shown, in this embodiment, when the local oscillator laser is in frequency scanning mode, the second photodetector 310 is used to detect the saturated absorption spectrum of hot atoms, and the first photodetector 24 is used to detect the laser after passing through the cold atom cluster and obtain the frequency modulation spectrum of the cold atoms. By calibrating the amplitude and peak-to-peak frequency interval of the cold atom frequency modulation spectrum, the required frequency modulation spectrum slope can be calculated. This slope serves as the conversion coefficient of voltage and frequency deviation in the subsequent voltage-controlled crystal oscillator 28, providing a quantitative basis for frequency feedback calibration.

[0066] like Figure 4 As shown, after the local oscillator laser completes pre-locking, the entire optical frequency standard system operates in a 500-millisecond cycle. In the first cycle, only the remaining error signal voltage is collected and its average value is calculated as the reference voltage value for subsequent feedback calibration; no frequency correction is performed. From the second cycle onwards (i.e., N=2, 3, 4, ...), the average voltage value collected 30 milliseconds after each cycle is compared with the reference value of the first cycle to obtain the frequency deviation of the current cycle. At the beginning of the next N+1 cycle, this frequency deviation is added as a correction to the fixed modulation frequency of the third acousto-optic modulator 19, thereby achieving closed-loop calibration control of the local oscillator laser frequency.

[0067] In this embodiment of the invention, a cold atom frequency modulation spectrum is used as a feedback reference to achieve closed-loop frequency calibration of the hot atom local oscillator laser. However, the invention is not limited to this detection method and can also be replaced with an optical path configuration using a cold atom modulation transfer spectrum to achieve the same feedback control function. The timing control module for timing control of the cooled laser source, the re-pumped laser source, and the data acquisition and processing module is developed and implemented based on the LabVIEW platform in this embodiment, but its specific implementation is not limited to this. It can also be implemented using other control software or hardware platforms with timing control functions, such as Python, MATLAB, and C++, as long as precise timing triggering and synchronous control of each component can be achieved.

[0068] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. The aforementioned substitutions include substitutions for different atoms and different wavelengths, such as replacing rubidium atoms with cesium atoms with diffuse reflection cooling at 852 nm, or replacing the 780 nm probe laser source with any other wavelength corresponding to atomic transitions, such as 420 nm. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection claimed by the present invention is determined by the scope defined in the claims.

Claims

1. A frequency control device for optical frequency standard based on diffuse reflection laser cooling of atoms, characterized in that, include: Cooling laser source, re-pumped laser source, beam combiner, distribution assembly, long strip vacuum glass tube, detection laser source, first electro-optic phase modulator, first photodetector, laser phase detection module, data acquisition and processing module, frequency control unit, frequency tuning unit, and timing control module; The cooling laser source outputs a cooling laser, the re-pumped laser source outputs a re-pumped laser, and after being combined by the beam combiner, they are distributed into multiple laser outputs by the distribution component. The elongated vacuum glass tube is a hollow cylindrical structure. Except for the two ends, the outer surface of the glass tube is coated with a diffuse reflection coating. The side of the cylinder is provided with one or more incident windows for introducing the laser to prepare cold atomic clusters inside the glass tube. The probe laser source outputs a pre-stabilized local oscillator laser, which is divided into an output beam and a probe beam. The probe beam is phase-modulated by the first electro-optic phase modulator and passes through the long strip vacuum glass tube. After interacting with the cold atoms inside the tube, it is received by the first photodetector and a detection signal is generated. The laser phase detection module is used to generate a modulation signal and demodulate the detection signal to generate an error signal; The timing control module is used to control the switching timing of the cooling laser source and the re-pumped laser source, and synchronously control the data acquisition and processing module to sample and statistically analyze the error signal and calculate the frequency deviation during the laser shutdown phase; The frequency control unit drives the frequency tuning unit according to the frequency deviation to perform closed-loop tuning of the local oscillator laser frequency, so as to realize the frequency control of the local oscillator laser by cold atoms.

2. The optical frequency standard frequency control device based on diffuse reflection laser-cooled atoms as described in claim 1, characterized in that, The detection laser source includes a detection laser, an isolator, a thermal atom gas chamber, a second electro-optic phase modulator, a second photodetector, and a laser phase detection and high-speed servo control circuit, used to generate and pre-stabilize the local oscillator laser; the thermal atom gas chamber inside the detection laser source is provided with a temperature control structure and a magnetic shielding structure on the outside.

3. The optical frequency standard frequency control device based on diffuse reflection laser-cooled atoms as described in claim 1, characterized in that, The distribution assembly includes a second polarizing beam splitter and a third polarizing beam splitter; a third half-wave plate and a fourth half-wave plate are provided on the laser path between the beam combiner and the distribution assembly to adjust the optical power distributed to each branch; each laser output end of the distribution assembly is provided with an optical fiber coupling head, and the optical fiber coupling head is connected to a one-to-two multimode optical fiber to guide the laser to the incident window of the long strip vacuum glass tube.

4. The optical frequency standard frequency control device based on diffuse reflection laser-cooled atoms as described in claim 1, characterized in that, The frequency control unit includes a voltage-controlled crystal oscillator, a second acousto-optic modulator, and a third acousto-optic modulator; the second acousto-optic modulator and the third acousto-optic modulator are disposed in the detection optical path, and their frequency shifting frequencies are the same but their directions are opposite.

5. The optical frequency standard frequency control device based on diffuse reflection laser-cooled atoms as described in claim 1, characterized in that, It also includes a first acousto-optic modulator, a first reflector, a first half-wave plate, and a second half-wave plate; the first acousto-optic modulator is disposed on the output path of the cooled laser source; the first reflector, the first half-wave plate, and the second half-wave plate are respectively used to guide the laser into the beam combiner and adjust its optical power; it also includes a fourth polarizing beam splitter for splitting the local oscillator laser into an output beam and a probe beam, and a fifth half-wave plate disposed in front of the fourth polarizing beam splitter; the probe laser source is also provided with a sixth half-wave plate and a seventh half-wave plate for adjusting the splitting ratio; it also includes a second reflector, a third reflector, and a fourth reflector; the second reflector is used to reflect the phase-modulated probe beam into the elongated vacuum glass tube; the third and fourth reflectors are disposed inside the probe laser source and are used to guide the laser through the hot atom gas chamber.

6. A method for controlling the frequency of an optical frequency standard based on diffuse reflection laser-cooled atoms, characterized in that, Includes the following steps: 1) A frequency-stabilized cooling laser is output through a cooling laser source, and a frequency-stabilized re-pumped laser is output through a re-pumped laser source. The cooling laser and the re-pumped laser are combined by a beam combiner, and then distributed into multiple lasers by a distribution component and guided into a long strip-shaped vacuum glass tube. Cold atomic clusters are prepared by multiple reflections of the multiple lasers in the glass tube. 2) The local oscillator laser, which is pre-stabilized by detecting the output of the laser source, is divided into an output beam and a probe beam; 3) The probe beam is phase-modulated by the first electro-optic phase modulator and guided to pass through the cold atom cluster. The probe beam after passing through the cold atom cluster is received by the first photodetector to generate a detection signal. The laser phase detection module generates a modulation signal and demodulates the detection signal, thereby generating an error signal that reflects the deviation of the local oscillator laser from the transition frequency of the cold atom. 4) The timing control module controls the switching timing of the cooling laser source and the re-pumped laser source, and synchronously controls the data acquisition and processing module to sample and statistically analyze the error signal during the laser shutdown phase to calculate the frequency deviation; the frequency control unit drives the frequency tuning unit set on the local oscillator laser path according to the frequency deviation to perform closed-loop calibration of the local oscillator laser frequency, thereby realizing the frequency control of the local oscillator laser by cold atoms.

7. The optical frequency standard frequency control method based on diffuse reflection laser-cooled atoms as described in claim 6, characterized in that, After the cooling laser output described in step 1) is first subjected to acousto-optic modulation for red detuning and frequency shifting, it is then combined with the re-pumped laser through the first polarization beam splitter. The re-pumped laser is used to maintain the continuous cooling state of cold atoms in the long strip vacuum glass tube. By adjusting the angle of the half-wave plate set at the front end of the distribution component, the power distribution ratio of the multiple lasers distributed to each incident window is precisely controlled. The distribution component includes a multi-stage polarization beam splitter, and each laser output end is connected to a one-to-two multimode fiber to guide the laser to multiple incident windows on the side wall of the long strip vacuum glass tube.

8. The optical frequency standard frequency control method based on diffuse reflection laser-cooled atoms as described in claim 6, characterized in that, In step 2), the thermal atom gas chamber inside the probe laser source is used to lock the probe laser at the atomic transition frequency through modulation transfer spectrum stabilization technology, thereby obtaining a pre-stabilized local oscillator laser.

9. The optical frequency standard frequency control method based on diffuse reflection laser-cooled atoms as described in claim 6, characterized in that, In step 3), before the probe beam passes through the cold atom cluster, it is first frequency-shifted by two acousto-optic modulators with the same frequency but opposite directions; the laser phase detection module filters, amplifies, and demodulates the detection signal output by the first photodetector to obtain the residual error signal reflecting the drift of the local oscillator laser relative to the cold atom resonance frequency; in step 4), the data acquisition and processing module converts the acquired error signal into a voltage deviation, and the frequency control unit is a voltage-controlled crystal oscillator used to convert the voltage deviation into a laser frequency deviation and feed it back to the acousto-optic modulator, which serves as the frequency tuning unit.

10. The optical frequency standard frequency driving method based on diffuse reflection laser-cooled atoms as described in claim 6, characterized in that, In step 4), a fixed duration is used as a complete operating cycle. The cooling and re-pumping light are turned on at the beginning of the cycle, and the light is turned off and error signal is sampled at the end of the cycle. The calculated frequency deviation is compensated to the frequency tuning unit at the beginning of the next cycle.

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